Interior paint comes in many shades and finishes, resists damage, and is easy to clean. But can you use interior paint outside?
Exterior paint differs from interior paint in a few ways. However, paint manufacturers have also greatly improved their indoor paint formulas.
What was once an easy rule of thumb - outdoor paint for outdoor painting - needs to be revisited in light of these changes.
Read on to learn more about interior paint and what to do if you accidentally use interior paint outside.
What Is The Difference Between Interior & Exterior Paint?
Exterior paint is designed to withstand outdoor elements like sunlight, moisture, and mold, while interior paint is meant to survive scuffs, scratches, and handprints.
Paint meant for outdoor use has a higher concentration of pigments that may not be organic like the ones in interior paint.
Interior paint is more likely to be water-based, also called latex paint. The solvent - the liquid that serves as a vehicle for the pigment - is just water, making it easier to clean than exterior paint.
The alternative is an oil-based paint with solvents like alcohol, linseed oil, esters, or other natural oils.
Oil-based exterior paint has higher volatile organic compounds (VOCs), making the fumes more harmful for a longer time. This is also a reason why exterior paint shouldn't be used inside.
Additives like anti-mold agents, agents to prevent fading, and weatherproofing agents are also put into exterior paint. Interior paint comes in a wider variety of finishes, while exterior paint is almost always a matte or flat finish.
Finally, exterior paints tend to cost more, especially the ones with special attributes.
This is the main reason why people are so curious about using interior paint on exterior surfaces; the money savings are admittedly tempting at first, particularly if you already have leftover interior paint.
Can You Use Interior Paint Outside? 4 Reasons Why You Can’t
Not so long ago, oil-based paint was the go-to for exterior painting because it binds to surfaces better and is more resistant to outdoor conditions.
Water-based paints have advanced to the point where you can use either. So can emulsion/latex paint be used outside?
Using interior paint outside is certainly possible, but it’s a bad idea. Here’s why:
1. Not Designed To Protect From Rain, Snow, Mold, Or Mildew
Whether it’s water- or oil-based, exterior paint has additives that prevent its color from fading and also keep weather moisture from damaging it.
Let moisture in or paint onto a wet surface, and you could get bubbles that lead to cracking and peeling.
Exterior paint has lots of synthetic additives and binders to help it withstand wet conditions and avoid chalking and breaking.
Mold and mildew can discolor paint and create a need for a new paint job all over again.
So even if you do save some money using interior paint outside, you’ll pay more on a new coat later.
2. More Porous & Has Lack Of Resin To Bind
Exterior paint is less porous, which is part of the reason why it’s better at handling moisture.
Oil-based paint is the least porous, and while it is bad to use oil-based paints on exterior surfaces like stucco and brick, they do bind better to metal and some other surfaces.
Still, even comparing water-based interior and exterior paints, interior paints are more porous because it makes them easier to clean, and they don't have to withstand bad weather.
Water-based paint is fast becoming the go-to variety because it has fewer fumes, it doesn’t have to be disposed of in a special way, and it's easier to use.
But water-based interior paints have more rigid resins to make them more durable against scrapes and scratches, making them more fragile when applied outdoors.
3. Increases Amount Of Work With Lower-Quality Finish
Sunlight is a big deal for exterior paint jobs. Some interior paint will dry so quickly that you’ll wind up being able to see marks in the finished paint job, even if you’re using a paint sprayer that atomizes and distributes paint fairly evenly.
Interior paint may also require more coats because of the weaker binding to exterior surfaces that we already mentioned. Plus, it’s not designed to handle UV rays and temperature changes. It will fade in sunlight, meaning you’ll wind up repainting sooner.
Although a paint sprayer will make the job easier, interior paint still creates more work for you.
It has to be scored with sandpaper to create better adhesion conditions if you want to use it as a primer, and you'll have to put a final coat of exterior paint or some kind of sealant if you want it to last.
4. Voids Paint Warranty
Some paint products come with extended or lifetime warranties that protect against defaults in the paint itself. However, if you don't follow manufacturer recommendations, i.e., using interior paint on exterior walls, then they won’t honor the warranty.
While the warranty is easy to back out of if the paint does last for years, there’s no reason to give the manufacturer an even easier reason to squirrel out of the deal.
Stick to exterior paint for exterior surfaces just in case there’s a defect with the paint. Even if it doesn’t come with a warranty, the paint job will be better if you use the right product.
Can I Make Interior Paint Into Exterior Paint?
Technically, learning how to make exterior paint is kind of the same as learning how to make interior paint into exterior paint.
By that, we mean you would have to procure additives to make interior paint waterproof, mold-resistant, and fade-resistant and then learn how to mix it to make it into exterior paint of the same quality as the manufacturer.
You shouldn't mix interior paint with exterior paint because you'd be diluting the exterior paint and reducing its durability and waterproofing.
Plus, unless you have exactly the same shade of paint, you could be corrupting your color by mixing two different varieties.
What Will Happen If You Use Interior Paint Outside?
If you’ve finished the whole job with interior paint, then the most likely outcome is that the paint will last a few years instead of 10 or more like it would have with exterior paint.
Depending on the surface you painted, interior paint could crack or fade even before that.
It’s not going to damage your house, but it’s certainly not going to last as long as it would have if you use exterior paint.
How long does interior paint last outside?
Most likely 2 - 5 years, although there are some miracle cases where weather conditions are fair enough for it to last longer.
Even if you live where the weather is always great, early fading is a problem you’re likely to face with interior paint on exterior surfaces.
What To Do If You Accidentally Used Interior Paint Outside
The best thing to do if you’ve already painted the outside of your house with interior paint is to repaint it or cover it with high-quality exterior paint.
Hopefully, you’ll realize the mistake before you’ve made it far on the project because you need to sand away some of the interior paint or prime over it to make sure you get a nice final coat with the exterior paint.
If you’ve used water-based paint on wood, it may swell. Any interior paint is likely to fade, crack, or chalk on an exterior surface, which will hurt the value of the house until you rectify the mistake. Better to just bite the bullet and repaint it while you’re in DIY mode.
Using Interior Paint Outside FAQs
Can I use interior primer outside?
Like interior and exterior paint, primers are manufactured to certain specifications. Indoor primers are made to be more adhesive and create a flat surface.
That’s great for indoor paint jobs, but it’s bad for outdoor ones. Plus, the primer won’t protect against weather conditions or mold.
Is it better to spray or brush exterior paint?
Some exterior paint may be thick and need to be thinned out a bit, but it can usually be used with a sprayer.
Paint sprayers make painting siding and uneven surfaces easier and help you cover a wider space in a shorter amount of time. Try an HVLP sprayer if you’re using paint with high VOCs.
Do I need to prime over old exterior paint?
If it’s an old oil-based coat, you definitely should. To find out, dip a washrag in some denatured alcohol and rub the paint. If it stays, it’s oil-based. If not, it’s water.
How many coats of paint should you put on a house exterior?
You’ll know when it looks right. Thinner paint and stains will allow you to see the surface of the siding or material beneath it - if that’s not what you’re aiming for, you might have to do two or three coats of paint.
What time of year is best to paint the exterior of a house?
In years past, painting when it was under 50°F was not possible. Now, you can paint in temperatures as low as 35° depending on the manufacturer’s specifications.
Still, exterior paint will dry faster in good weather, so spring, summer, and early fall are the best times of year to do exterior painting.
Try to avoid using interior paint on exterior surfaces. It might seem like a good idea if you have lots of paint left over from remodeling the inside of your house, but you'll regret it eventually.
Interior paint isn't made to withstand sunlight, rain, or mold, so it's best not to use it outside.